1977 Piper PA-32-300 accident at Hedingen (HB-OMB)

Accident overview and aircraft details On 22 July 1977, a Piper PA-32-300 with registration HB-OMB, operated by Motorfluggruppe Zürich (MFGZ), crashed during a commercial flight near Hedingen, Switzerland. The aircraft was powered by a Lycoming IO-540-K1A5 engine and equipped with Bendix S 1200 magnetos. The pilot, born in 1927, held a valid commercial pilot license and had 994 total flight hours, including 319.5 hours on the accident type. The aircraft had a total operating time of 1,572.51 hours at the time of the accident. ## Flight sequence and emergency landing The pilot departed Zurich with five passengers for a scenic flight around the Titlis. Approximately four to five minutes after takeoff, at an altitude of about 1,000 meters near Birmensdorf, the pilot noticed slight engine vibration, followed immediately by engine failure. After unsuccessful attempts to restart the engine, the pilot selected a nearby field northwest of Hedingen for an emergency landing. The terrain was hilly, overgrown, and built-up, making it unsuitable for a forced landing. During the final approach, the pilot discovered a high-voltage power line and a stream crossing the field. He successfully flew under the power line and over the stream, touching down in the field about 1.5 meters after a field path. The landing gear sank into the soft ground and was torn off, causing the aircraft to stop approximately 32 meters from the touchdown point after a left roll. ## Investigation findings and probable cause The pilot sustained minor injuries, while all five passengers were unharmed. The aircraft was destroyed, with minor third-party damage. Investigation revealed that the engine failure was caused by a broken pawl in the left magneto snap coupling. A broken fragment jammed the camshaft drive, preventing the camshaft from turning and causing the valves to stop operating, which led to immediate engine failure. The broken pawl was caused by abnormal wear in the left magneto snap coupling system due to excessive play. The exact cause of the abnormal wear could not be determined, though material defects, insufficient monitoring, or inadequate manufacturer maintenance procedures were considered. Following the accident, Bendix issued a service bulletin in March 1978 requiring inspection of affected magnetos for abnormal wear at 1,000 hours, which was later adopted as an airworthiness directive by the U.S. Federal Aviation Administration and the Swiss Federal Air Office.